The present invention relates to the art of diagnostic imaging. It finds application in conjunction with single-photon emission computed tomography (SPECT) with multi-headed cameras and will be described with particular reference thereto. It is to be appreciated, however, that the invention will also find application in other non-invasive investigation techniques such as positron emission tomography (PET) and other diagnostic modes in which a subject is examined for emitted radiation and with transmitted radiation.
Heretofore, single photon emission computed tomography has been used to study a radionuclide distribution in subjects. Typically, one or more radiopharmaceuticals are injected into a subject. The radiopharmaceuticals are commonly injected into the subject's blood stream for imaging the circulatory system or for imaging specific organs which absorb the injected radiopharmaceuticals. Gamma or scintillation camera heads are placed closely adjacent to a surface of the subject to monitor and record emitted radiation. In single photon-emission computed tomography, the head is rotated or indexed around the subject to monitor the emitted radiation from a plurality of directions. The monitored radiation data from the multiplicity of directions is reconstructed into a three dimensional image representation of the radiopharmaceutical distribution within the subject.
One of the problems with the SPECT imaging technique is that photon absorption and scatter by portions of the subject between the emitting radionuclide and the camera head distort the resultant image. One solution for compensating for photon attenuation is to assume uniform photon attenuation throughout the subject. That is, the subject is assumed to be completely homogenous in terms of radiation attenuation with no distinction made for bone, soft tissue, lung, etc. This enables attenuation estimates to be made based on the surface contour of the subject. 0f course, human subjects do not cause uniform radiation attenuation, especially in the chest.
In order to obtain more accurate radiation attenuation measurements, a direct measurement is made using transmission computed tomography techniques. In this technique, radiation is projected from a radiation source through the subject. Radiation that is not attenuated is received by detectors at the opposite side. The source and detectors are rotated to collect transmission data concurrently with the emission data through a multiplicity of angles. This transmission data is reconstructed into an image representation using conventional tomography algorithms. The radiation attenuation properties of the subject from the transmission computed tomography image are used to correct for radiation attenuation in the SPECT or other emission data.
There are several known methods which correct distortions in single photon emission computed tomography (SPECT) images caused by spatially varying collimator geometric point response and scatter. These methods additionally correct for distortions caused by attenuation. The distortions are corrected by using an iterative reconstruction algorithm which requires one projection and one backprojection operation per iteration. The attenuation, geometric response, and scatter are modeled in the projector/backprojector. A problem with these known methods is that the three-dimensional implementation of collimator response and scatter is computationally very expensive.
For parallel geometry, the computational speed is improved by using a projector/backprojector pair that rotates the image volume for each projection angle. In this manner, the projection operation can be achieved by a simple weighted summation, and the backprojection operation can be achieved by copying weighted projection array values to the image volume.
The present invention contemplates a new and improved projector/backprojector for converging-beam geometries.